Hot melt adhesives, which attract attention as solvent-free adhesives with no VOC emissions, contain polymers called base polymers. Known such base polymers are ethylene-based copolymers such as ethylene/vinyl acetate copolymers (EVA) and ethylene/α-olefin copolymers, amorphous polyolefins (APAO) and styrene elastomers (SBS, SIS, SEBS and SEPS). As known in the art, ethylene/vinyl acetate copolymers (EVA) and ethylene/α-olefin copolymers are excellent in flexibility but are inferior in heat resistance and rubber elasticity; styrene elastomers (SIS and SBS) have good flexibility and rubber elasticity but are poor in heat resistance and thermal stability; and amorphous polyolefins (APAO) are highly heat stable but are inferior in mechanical properties (strength). Few base polymers satisfy all the properties with good balance. SEBS is known as a material that is relatively excellent in balance of these properties, but its heat resistance is sometimes insufficient when the resin is used under severe conditions. In particular, stress at a high temperature often results in cohesive failure of adhesives (which is attributed to insufficient heat resistance of the base polymer). Thus, there has been a demand from the industry for base polymers having enhanced performances.
A variety of novel base polymers have been recently proposed which use soft polypropylene polymers having excellent heat resistance. Such soft polypropylene polymers have characteristics that the materials have a narrow molecular weight distribution and a narrow composition distribution by the use of a specific catalyst such as a metallocene catalyst. It has been disclosed that they show excellent performances (for example no reduction in bonding properties due to low-molecular weight components, and no deterioration in processability due to high-molecular weight components) compared to known products having a wide molecular weight distribution and a wide composition distribution, for example APAO.
Patent Literature 1 discloses a technology of obtaining a hot melt adhesive using a propylene/ethylene copolymer. The propylene/ethylene copolymer has excellent flexibility, rubber elasticity and thermal stability, but is insufficient in heat deformation temperature and creep resistance properties under stress at high temperatures. The patent literature also discloses that a crystalline isotactic polypropylene component is added to improve heat resistance. However, according to the knowledge of the present inventors, propylene/ethylene copolymers have poor compatibility with polypropylenes and therefore the improvement in heat resistance is probably insufficient.
Patent Literature 2 discloses a technology of obtaining a hot melt adhesive using a soft polypropylene polymer in which flexibility is achieved by controlling the stereoregularity of the propylene units. This soft polypropylene polymer is considered to have good compatibility with crystalline isotactic polypropylenes compared to the propylene/ethylene copolymers described in Patent Literature 1. However, according to the knowledge of the present inventors, the glass transition temperature of the polymer is probably high, around room temperature, because comonomers other than propylene are little. Accordingly, the base polymer will embrittle at low temperatures and the adhesive will lose stress relaxation properties and will separate easily. Copolymerizing such polymers having low stereoregularity with comonomers such as ethylene will be a possible approach, but in this case the polymers have almost no crystallinity and consequently mechanical strength is lost. Further, the compatibility with crystalline isotactic polypropylene is lowered at the same time, and consequent lowering in heat resistance is anticipated.
Patent Literatures 3 and 4 disclose technologies of obtaining hot melt adhesives having excellent adhesion, wherein a substantially completely amorphous soft polypropylene polymer which has no heat of crystal fusion is used. However, according to the knowledge of the present inventors, the addition of crystalline isotactic polypropylenes to amorphous soft polypropylene polymers showing no crystallinity does not provide sufficient heat resistance, and good mechanical strength and creep resistance properties will not be achieved in particular at high temperatures.
Patent Literature 5 discloses a technology of adhesive compositions using a propylene/1-butene random copolymer having high stereoregularity. Such propylene/1-butene random copolymers have good compatibility with crystalline isotactic polypropylenes compared with the propylene/ethylene copolymers. However, they are very rigid and will not show flexibility required for hot melt adhesives.